Permeability determination from NMR relaxation measurements for fluids in porous media

ABSTRACT

The present invention is a method to rapidly determine the fluid-flow permeability of porous media with nuclear magnetic resonance (NMR). The method can be applied to measurements of permeability in fluid-saturated earth formations using NMR logging tools.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation-in-part of U.S. Ser. No. 763,026 filed Sept. 20, 1991 now U.S. Pat. No. 5,289,124.

BACKGROUND OF THE INVENTION

The present invention relates to a method for determining the fluid flow permeability of porous media. In particular, the present invention relates to measuring the permeability by nuclear magnetic resonance (NMR) using the T₂ relaxation time for the decay of the transverse magnetization of fluids saturating the medium under investigation.

The properties of fluid in porous media are of great relevance in many fields of science and engineering. There are numerous measurements which bear some importance on characterizing fluid properties in confined geometries like sandstone rocks. We list here a few: porosity, fluid flow permeability (both dc and ac), electrical conductivity, wettability, etc. Quantities like porosity and fluid flow permeability in porous rocks are of great relevance for determining, the producibility of petroleum reservoirs.

For porous media it has become customary to speak of the solid material which forms the "backbone" as the matrix and its complement as the pore space. Porosity is defined as the ratio of pore space volume inside the porous material to the total volume of the porous medium. Permeability is a measure for the ability of porous materials like e.g. porous rocks to permit fluid flow through the pore space. It generally increases with porosity, but also depends on other parameters of the rocks as e.g. the specific surface area of the pore space, the pore size distribution and the pore shape. The fluid flow permeability can vary by about 8 orders of magnitude in loose sediments and sedimentary rocks. It has the dimension of area and is defined by Darcy's law which relates the rate of fluid flow to the pressure differential between two parallel planes for inflow and outflow. The fluid flow permeability is measured in the laboratory by fitting sleeves to core samples which are often cylindrically shaped. The top and bottom of the core samples are connected to fluid inlets and outlets and a known pressure difference is applied across the sample. The fluid flow rate is measured for a set of different pressure gradient. Liquids or gases can be used as flowing medium, although the measurement using a liquid is generally easier as in most cases the liquid can be considered incompressible. The laboratory procedure therefore requires first to drill core plugs from core samples, which have to be cleaned with various solvents. In contrast the method of the present invention can be carried out with a nuclear magnetic resonance logging tool to measure in situ the transverse relaxation time of the fluids saturating an earth formation to accurately predict the fluid flow permeability of the earth formation.

Nuclear magnetic resonance (NMR) has been employed for some time to study fluids permeating the pore space of porous media [see J. R. Banavar and L. M. Schwartz, "Molecular Dynamics in Restricted Geometries", chapter 10, edited by J. Klafter and J. M. Drake, J. Wiley (1989)]. The fluid supplies the probe particles which diffuse in the pore space. Since the classic paper by Brownstein and Tarr (BT) [see K. R. Brownstein and C. E. Tarr, Physical Review A, 19, 2446(1979)] it has been realized that nuclear spin relaxation can provide information about the pore space geometry. BT discussed the case of T₁ and T₂ relaxation in an isolated pore where the nuclear spins are relaxed by collisions with the pore walls. The interpretation of T₁ measurements with this model for fluids in porous media can present several problems. In the limit where the nuclear spins diffuse at a fast rate to the pore surface and the surface relaxation is in comparison relatively slow, the averaged relaxation curve can be related to the pore size probability distribution. In this so called fast diffusion limit where the lowest order relaxation mode dominates one still has to assume that the surface relaxation strength is uniform and the pores are isolated to relate the distribution of relaxation times uniquely to the pore size distribution. It is conceivable to have porous samples with the same pore size geometry but different levels of paramagnetic impurities which influence the surface relaxation velocity while the fluid flow permeability would remain unchanged. To obtain a reliable estimate of the fluid flow permeability with NMR one therefore has to perform an experiment which directly probes fluid transport in the porous medium like for example the diffusion of fluid molecules in the pore space. For T₁ measurements the nuclear spin relaxation depends on the rate at which magnetization is carried to the surface but also on the surface relaxation velocity ρ. As the surface relaxation strength ρ has no bearing on permeability one can therefore hope to correlate T₁ and the fluid flow permeability only for classes of materials with similar surface relaxation properties.

There is an increasing interest in applying NMR in well-bore environments to determine the properties of fluid carrying earth formations [see P. N. Sen, C. Straley, W. E. Kenyon and M. S. Whittingham, Geophysics, 55, 61-69(1990)]. This interest has been spurred by the introduction of a new generation of NMR logging tools by NUMAR [see M. N. Miller, A. Paltiel, M. E. Gillen, J. Granot and J. C. Brouton, Society of Petroleum Engineers, SPE 20561, 321(1990)], which are already being used in the field. The new NMR logging tools are very well fitted to carry out the physical measurements required for our method of invention.

In the present invention, a measurement of the transverse relaxation time T₂ for fluids in porous media is used to determine the permeability of the medium by taking advantage of magnetic field inhomogeneities across pores. For strong magnetic fields and in the fast diffusion limit the relaxation is determined to first order by the transport of magnetization through the pore space and not the surface relaxation velocity. It will be shown that it is possible to correlate T₂ to a length characteristic of the pore space geometry which can also be determined independently from mercury injection experiments and thereby relate T₂ to the fluid flow permeability. It is also feasible to study the degree to which the diffusion of fluid molecules is restricted by the pore space geometry. T₂ for fluids in porous media is in general orders of magnitude shorter than T₁ in marked contrast to the situation for bulk fluids. The main mechanism for T₂ relaxation of the fluid spins in strong magnetic fields is due to the internal random magnetic field gradients generated by the difference in magnetic susceptibility for the fluid filling the pore space and the material making up the matrix of the porous medium. At low fields surface relaxation can not be be neglected but the τ dependence of T₂ (τ) is still primarily due to diffusion in the internal magnetic field gradients. Surface relaxation will under standard experimental conditions not lead to a τ dependence of T₂ in a CPMG experiment. This is confirmed by recent experimental results at low field with an NMR logging tool[see M. N. Miller, A. Paltiel, M. E. Gillen, J. Granot and J. C. Brouton, Society of Petroleum Engineers, SPE 20561, 321(1990)]. The spatial dependence of the internal gradients is determined by the pore space geometry and pore size distribution. The internal gradients in turn determine the rate at which the spins diffusing through the pore space loose their phase memory. The loss of phase memory can be monitored with a multi spin-echo pulse sequence like the Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence [see S. Meiboom and D. Gill, Rev. Sci. Instr., 29, 688 (1958)].

The phenomenon of spin echoes essential to the present invention was first discovered in NMR by Erwin Hahn [see E. L. Hahn, Phys. Rev., 77, 297 (1950)]. In an inhomogeneous magnetic field nuclear spins will precess at a Larmor frequency, ν_(L), determined by the local field. After an initial radiofrequency pulse which tips the spins into a plane transverse to the direction of the applied static magnetic field the spins are all in phase and the sum of the total transverse magnetization is at the maximum possible value. Due to the spread in precession frequencies the spins will dephase and the macroscopic magnetization measured with the NMR instrument will decay. It is useful to remember here that the macroscopic magnetization is a vector sum of the magnetic moments of the spins which vanishes when the phases of the magnetic moments are random. One can reverse the dephasing process by applying a 180 degree pulse a time τ/2 after the initial radio-frequency pulse which tipped the nuclear spins into the transverse plane. Immediately after this pulse a spin which precesses at a faster frequency than the average lags behind by an angle which is exactly the same angle by which it was ahead of the average immediately before the 180 degree pulse. Similarly spins precessing at a frequency slower than the average are now ahead. A time τ/2 after the 180 degree pulse the spins will be again be in phase and one can observe a spin-echo. Spins diffusing will be subject to different local fields between the time the first pulse was applied and the detection of the spin echo. As their Larmor frequency is not constant the refocusing of magnetization will be incomplete and the echo will be attenuated. The degree of attenuation depends on the displacement and field inhomogeneity. This attenuation can be used to measure diffusion constants in fluids and to probe the diffusion of fluid spins in the pore space of porous media.

SUMMARY OF THE INVENTION

The present invention provides a method for determining the permeability of porous media saturated with a liquid using nuclear magnetic resonance (NMR). The steps of the method include: (a) applying a radiofrequency pulse sequence which after an initial pulse generates successive spin echoes with a train of radio frequency pulses spaced apart by a time interval of length τ wherein all pulses have a carrier frequency corresponding to the Larmor frequency of the fluid spins filling the pore space of the medium for which the fluid flow permeability is to be determined; (b) measuring the decay of the transverse magnetization at each of the successive regularly spaced midpoints between the 180 degree pulses where the midpoints coincide with the peak of the spin echoes; (c) repeating steps a and b at least one more time wherein each repeat of step (a) uses said radio frequency pulse train with a different value of the pulse spacing τ; (d) determining the transverse relaxation time T₂ (τ), from the transverse magnetization decay for each value of τ and determining one of a prefactor Δ or a restricted diffusion length l_(nmr) from said T₂ (τ); (e) measuring the porosity of said porous media; (f) determining the permeability of said media from the porosity and either a prefactor Δ or a restricted diffusion length l_(nmr) from said T₂ (τ).

In a preferred embodiment the method is performed as a down-hole wellbore measurement to swiftly and accurately determine the fluid-producing potential of an earth formation using a magnetic resonance logging system which employs static and radio-frequency magnetic fields to perform the spin-echo CPMG pulse experiment in a wellbore environment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a shows pore size distribution calculated with equation 11;

FIG. 1b shows the numberically evaluated CPMG decay for a certain τ value and the pore size distribution shown in (1a);

FIG. 1c shows the T₂ values calculated from computer simulated CPMG decays for the same pore size distribution.

FIG. 2 shows a fit of T₂ (τ) for a restricted diffusion model.

FIG. 3 shows a plot of the restricted diffusion length l_(nmr) versus l_(c) obtained from mercury injection experiments.

FIG. 4 shows the values of T₂ (τ→0) obtained from numerical calculations for a set of pore size distributions versus the width Δ of the pore size distribution.

FIG. 5: Schematic diagram of the NMR spin-echo pulse sequence used for the measurement of T₂.

FIG. 6 shows an example of stretched exponential fit to CPMG decay for a water imbibed sandstone sample.

FIG. 7 shows the dependence of T₂ on CPMG pulse spacing τ for a set of sandstone samples. The T₂ values can be extrapolated to T₂ (τ→0) with an exponential function.

FIG. 8 shows a graph of extrapolated value of T₂ versus l_(c) determined from mercury injection for sandstone rock samples.

FIG. 9 shows a graph of T₂ (τ=0)×φ (φ is the porosity) versus permeability k for sandstone rock samples.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is a method to determine the permeability of porous media using pulsed nuclear magnetic resonance. This method uses the relaxation decay of the transverse component of the magnetization (T₂ relaxation) measured with the Carr-Purcell- Meiboom-Gill (CPMG) pulse sequence.

It can be shown (see below) that the magnetization as a function of time, t is described by a stretched exponential decay function of the form:

    m(t)∝ exp[-(t/T.sub.2).sup.62 .sbsp.2 ],            (1)

where β₂ is the stretch exponent. A nonlinear least squares fit of the measured values of magnetization m(t) to the stretched exponential function determines the relaxation time T₂ and the stretch exponent, β₂. If the pulse spacing, τ, is changed in incremental steps, a set of stretched exponential curves is produced yielding different values for T₂ (τ). For the case of free diffusion, the pore size distribution introduces a spectrum of T₂ decay values because the internal magnetic field gradient is related to the pore size. The analysis of the transverse magnetization decay in this case yields an effective T₂, which is a weighted average over the range of T₂ values. In this case a characteristic length can be determined from the prefactor of the functional form that describes the τ dependence of T₂ (τ). When T₂ is only measured for a limited range of τ values this prefactor is dominated by the pore size distribution and is not very sensitive to the exact functional form of T₂ (τ). A characteristic length, Δ , is derived from this prefactor.

    T.sub.2 (τ)∝ Δ/F(τ)                   (2)

where F(τ) is a fitting function which reproduces the τ dependence of T₂. Specific forms for F(τ) are discussed in the section on Theoretical Background.

For a sufficiently wide range of τ values, the exact functional form of T₂ (τ) is determined by the cross-over from free to restricted diffusion. For a given range of τ values, this cross-over is also dependent on the pore size distribution. Analysis of the functional form of T₂ (τ) in the region of this cross-over yields a characteristic length, l_(nmr), according to: ##EQU1## In either case, the characteristic lengths, Δ or l_(nmr), derived from the decay of T₂ (τ) with τ, are a measure of the fluid flow permeability. With l_(nmr) the fluid flow permeability is determined with

    k ∝ l.sub.nmr.sup.n φ.sup.m                     (4)

If a prefactor Δ has been determined from T₂ (τ) the fluid flow permeability is determined with:

    k ∝ Δ.sup.n φ.sup.m                       (5)

If T₂ (τ) is a simple exponential then the prefactor Δ and the extrapolated value of T₂ (τ) are related by T₂ (τ→0) ∝Δ. The value of T₂ extrapolated to τ=0 combined with the porosity, φ, of the material gives the permeability, k, of the porous medium according to (see FIG. 9):

    k ∝ [T.sub.2 (τ→0)].sup.n φ.sup.m.   (6)

The permeability relations 4, 5 and 6 follow from the relations: k ∝ l_(c) ² φ² and Δ∝ l_(nmr) ∝ l_(c) and T₂ (τ→0) ∝Δ. The proportionality constants in these permeability relationships 4, 5 and 6 can be determined from a calibration experiment with a porous material of known porosity and permeability. The exponents n and m may vary depending on which of the above relations 4, 5 and 6 is used to determine the permeability. The values of n and m are not necessarily the same for each of the relationships. The porosity is determined by first running an NMR experiment with a water jacket which surrounds the radiofrequency probe (e.g. of the NMR logging tool) and therefore calibrates the NMR signal for 100% porosity. To prevent the interference from local radio signals one can enclose the probe and water jacket in a Faraday cage. As the NMR signal is directly proportional to the number of hydrogen nuclei in the sensitive volume of the probe it will scale linearly to lower porosities. Only the contribution from liquid phases or non-adsorbed fluid will be recorded if the dead time of the instrument is set accordingly. The temperature of the fluid in the sensitive volume is of secondary influence. In a bore hole environment this varies with depth and can be compensated for by measuring the bottom temperature and temperature gradient. The NMR signal amplitude changes as a function of temperature as 1/T.

THEORETICAL BACKGROUND

The magnetization of the fluid spins diffusing in the pore space satisfies the following modified Bloch equation: ##EQU2## T_(b) is the bulk relaxation rate of the fluid in the pore space and D is the diffusion constant which is on the order of 2×10⁹ [m² /sec] for water at room temperature. The boundary condition is:

    Dn·∇M+ρM|.sub.surface =0.   (8)

where ρ is the surface relaxation velocity, which has units of length over time and can be thought of as the relaxation rate at the surface multiplied by the thickness of the layer of fluid spins relaxing near the surface. The bulk relaxation rate can always be factored out of the solution.

    M(r,t,)=m(r,t)exp(-t/T.sub.b)                              (9)

The Bloch equation becomes: ##EQU3## Brownstein and Tarr [K. R. Brownstein and C. E. Tarr, Physical Review A, 19, 2446-2453 (1979)] expressed the general solution as a sum of normal modes: ##EQU4## Two limiting cases can be considered for the magnetization decay which are characterized by decay constants τ_(c) : ##EQU5## In the fast diffusion case the magnetization is approximately uniform across an isolated pore and only the lowest mode in the above eigenmode expansion contributes significantly to the magnetization decay. In this limit, ρr/D<<1, and when the pores are approximately isolated (narrow throat limit) one obtains for m(t): ##EQU6## S_(p) and V_(p) are the surface area and the volume of a pore and V_(p) /S_(p) ≈r. For a pore size distribution the relaxation is described by: ##EQU7##

r_(c) is a lower cut-off on the pore size distribution. Thompson et al [see A. H. Thompson, S. W. Sinton, S. L. Huff, A. J. Katz, R. A. Raschke and G. A. Gist, Journal of Applied Physics, 65, 3259 (1989)] found that the pore size distribution of many porous rocks is well represented by the following class of functions:

    P(r)dr ∝ exp [-(r/Δ).sup.β/(1-β) ]dr(15)

and where Δ is a measure of the width of the pore size distribution and β lies between 0 and 1. A β value of 2/3 will yield a Guassian pore size distribution. This type of pore size distribution will lead to a stretched exponential decay in the fast diffusion limit:

    m(t) ∝ exp [-(t/T.sub.1.2).sup.β.sbsp.1.2 ].   (16)

This result is obtained by using the saddle point method on the integral in equation 14, i.e., we determine the pore size radius for which the exponent of the integrand goes through a maximum as a function of pore radius r.

Equation 16 is of a form observed experimentally for T₁ and T₂ magnetization decays for fluids in porous media for a broad set of experimental conditions, although it was derived for the case of surface induced relaxation. We wish to arrive at an expression which explicity accounts for the contribution to the T₂ decay from diffusion of fluid spins in porous media in the presence of internal magnetic field gradients. The magnetic field gradients lead to dephasing of the nuclear spins which can only be partially compensated with a spin-echo sequence as they diffuse. As a starting point we use an expression derived by Robertson and Neuman and experimentally confirmed by Wayne and Cotts for the decay of the transverse magnetization in the presence of a uniform gradient G [see B. Robertson Physical Review, 151(1), 273 (1966), R. C. Wayne and R. M. Cotts, Physical Review, 151(1), 263 (1966), C. H. Neuman, The Journal of Chemical Physics, 60 (11), 4508 (1974)]. Neuman produced expressions for the case where the spins are assumed to diffuse in a bounded medium of spherical geometry and the Carr-Purcell spin-echo sequence with spacing of the π pulses given by τ is being used: ##EQU8## θ=2τD/r² and the α are determined from tan α_(i) =2α_(i) /(2-α_(i) ²) whose solutions asymptotically approach: α_(i) =i·π. For small π the above expression agrees with the well-known expression for transverse relaxation of spins due to unrestricted diffusion in a uniform gradient:

    m(t)=m.sub.o exp [-γ.sup.2 G.sup.2 τ.sup.2 Dt/12].(18)

Since porous media are characterized by a distribution of pore sizes the internal gradients should be parametrized in terms of pore size length. To this end we make the reasonable assumption that the magnetic field gradient across a pore of radius r is inversely proportional to the pore radius:

    G=μ.sub.0 H.sub.o Δχ/r                        (19)

Δ.sub.χ is the susceptibility difference, μ_(o) the magnetic permeability of vacuum and H_(o) the magnetic field strength. This means that under this model the gradient is uniform over individual pores but varies from pore to pore. The contribution of the spins in each pore has to be weighted by the volume of the pore. For a pore size distribution we must average the magnetization decay, m(t), over the pore size distribution in a fashion similar to the example in equation 14. We therefore arrive at the following expression for the contribution to the transverse magnetization of the spins diffusing in random internal gradient fields: ##EQU9## where the last exponential explicitly accounts for the possibility of surface relaxation. This expression is the basis for numerical calculations performed for a series of pore size distributions. This expression was also previously, independently derived by Kleinberg and Horsfield [see R. L. Kleinberg and M. A. Horsfield, Journal of Magnetic Resonance, 88, 9-19 (1990)]. We numerically calculated the CPMG echo decay curve with the sum over the roots ∝_(i) being approximated up to the 20th term. The pore size distribution is of the form shown in Equation 15. The simulations were carried out in the same manner as the experiment. The CPMG decays were calculated for a set of values of τ. We observe that the calculated magnetization decays are well described by stretched exponential functions. This suggests that equation 16 applies to a broader class of relaxation decays than suggested by the derivation which only considered surface induced relaxation. Indeed the numerical results show that the decay of the magnetization due to diffusion in the presence of random magnetic field gradients and averaged over a pore size distribution also leads to stretched exponential decays.

FIG. 1a shows the pore size distribution calculated from equation 15 for typical values of Δ and β. FIG. 1b shows the magnetization decay calculated for the pore size distribution of 1a from equation 20. This calculation was repeated for several values of τ and the nonlinear least squares fits to the stretched exponential function yield values for T₂ and β₂. FIG. 1c shows the T₂ values thus obtained for a set of τ values.

Restricted diffusion can be expected at longer values of τ. The diffusion dynamics were therefore studied by systematically varying the parmameter τ in the CPMG sequence. The crossover from free to restricted diffusion will then be evident in the dependence of both T₂ and β₂ on the diffusion time set by τ. CPMG experiments were repeated for a series of τ values. Values for T₂ and β₂ were obtained by non-linear least squares fits to a stretched exponential function for each coherence decay. For free diffusion is a linear gradient G the CPMG T₂ ⁻¹ should be ∝ (γG)² Dτ², where D is the diffusion constant. For restricted diffusion D becomes effectively time dependent. To explain the τ dependence of T₂ we use a model which interpolates between the limits of free diffusion considered above and restricted diffusion at long τ. The cross-over from unrestricted to restricted diffusion is well characterized by:

    D.sub.eff =l.sub.nmr.sup.2 [1-exp(-D.sub.o τ/l.sub.nmr.sup.2)]/τ,(21)

a result which was derived first in a pioneering paper by Stejskal on NMR measurements of restricted diffusion [E. O. Stejskal, Journal of Chemical Physics, 43, 3597-3603 (1965)]. l_(nmr) should be on the order of V_(p) /S_(p), the volume-to-surface ratio of a pore. This gives a τ dependence for T₂ of the form: ##EQU10## This approximation for T₂ (τ) leads to the correct limiting behavior for τ→0. The experimental data points for T₂ (τ)⁻¹ fit well to the above equation. We observe that for typical parameters. 4D_(o) τ/l_(nmr) ² <<1. A power series expansion of equation 22 is therefore adequate as an approximation for T₂ (τ): ##EQU11##

The fit of equation 23 or 22 to the experimental T₂ (τ) data for several sandstone rocks are shown in FIG. 2. The fits to the experimental data yield coefficients for the two terms in the expression for T₂ (τ)⁻¹ of opposite sign as suggested by equation 23. The resulting values for l_(nmr) are seen in FIG. 3 to correlate with a quantity l_(c) derived from mercury injection. l_(c) provides an estimate of the fluid flow permeability [see A. H. Thompson, S. W. Sinton, S. L. Huff, A. J. Katz, R. A. Raschke and G. A. Gist, J. Appl. Phys., 65(8), 3259 (1989)]. Therefore since l_(c) is proportional to l_(nmr), l_(nmr) provides an estimate of the fluid flow permeability. Mercury injection data indicate that the exponents n and m in equation 4 should be on the order of 2.0 .

Equation 22 is one approximation to the crossover from free to restricted diffusion and other expressions have been proposed in the literature. All have in common that they provide a characteristic length l_(nmr) which is of relevance to determine the transport properties of the porous medium. The restricted diffusion analysis presented above is therefore not confined to the use of equation 22 but can be used for a variety of models describing the crossover from free to restricted diffusion. An example of an alternative model is equation 20 (used for numerical calculations), where the sum in the exponent can be used to define an effective diffusion coefficient. In each case the crossover from free to restricted diffusion is probed by measuring the τ dependence of T₂ (τ) as outlined in this invention.

PREFACTOR ANALYSIS

In actual practice the ability to determine the exact functional form which describes the τ dependence of T₂ (τ) is limited by the time available for signal collection. This limits both the range of τ values which can be measured and the signal-to-noise ratio of the data. However in each case there is a characteristic length which determines the dominant contribution to T₂ (τ). This length is related to the mean life time <t> for the magnetization decay. The mean life time is defined by [see D. J. Wilkinson, D. L. Johnson and L. M. Schwartz", Phys. Rev. B, 44, 4960-4973 (1991)]: ##EQU12## For stretched exponential decays the mean life time and T₂ from a nonlinear least squares fit to a stretched exponential are simply related: <t>=T₂ Γ(1/β₂) and for the range of β₂ values typical for magnetization decays in rocks the dependence of the mean life time on β₂ is weak. Using the approximation of free diffusion and with the assumption that the magnetic field gradient is inversely proportional to the pore radius the mean life time for the transverse magnetization decay is given by: ##EQU13## where δω=γGr ≈γΔχH_(o). The free diffusion expression is a good approximation when T₂ is measured only for a limited set of short τ values. Using equation 15 for the pore size distribution and a change of variable ((r/Δ).sup.β/(1-β) =x), the mean lifetime is given by: ##EQU14## Significantly the mean life time and therefore also T₂ are directly proportional to Δ and only weakly dependent on β and r_(c) /Δ for typical values of these two parameters. This relation between T₂ and Δ is a consequence of the pore size distribution. This result remains unchanged if we generalize the expression for the mean life time to include a different τ dependence for T₂ (τ). This is evident by substituting a general functional dependence T₂ ∝ F(τ) in place of the T₂ ∝ τ-² in the above integral. ##EQU15##

Empirically we have observed that at a magnetic field of 7T and for the range of experimental τ values used, the exponential function is a good approximation for F(τ). At other magnetic field strengths the simple exponential may not necessarily be a good approximation for F(τ. For the numerical simulations, we repeated the calculation of the T₂ (τ) data set for different pore size distributions where we arbitrarily changed Δ and β to change the shape of the pore size distribution. An exponential dependence of T₂ (τ) is seen to be also an excellent approximation for the numerically calculated results for T₂ (τ) as is evident from the example of FIG. 1c. For this case of a simple exponential dependence of T₂ (τ), the prefactor of the exponential is equivalent to the value of T₂ extrapolated to τ=0. Thus the exponential extrapolation of T₂ to τ=0 is a measure of the prefactor of the exponential. We therefore expect that the extrapolated value of T₂ to τ= 0 is proportional to Δ and this is confirmed by numerical simulations as shown in FIG. 4.

The procedure of estimating Δ from the τ dependence of T₂ either by prefactor analysis or extrapolation provides a simple estimate of the fluid flow permeability:

    k ∝ Δ.sup.m φ.sup.n                       (28)

The proportionality of T₂ (τ→0) to Δ (and therefore l_(c)) allows one to use T₂ (τ→0) to predict the fluid flow permeability from NMR measurements:

    k ∝ T.sub.2 (τ→0).sup.m φ.sup.n      (29)

It can be shown using pecolation theory [see A. H. Thompson, S. W. Sinton, S. L. Huff, A. J. Katz, R. A. Raschke and G. A. Gist, J. Appl. Phys., 65(8), 3259 (1989)] that Δ ∝ l_(c) and l_(c) is the characteristic length obtained from mercury injection. Katz and Thompson [see A. J. Katz and A. H. Thompson, Phys. Rev. B, 34, 8179-8181 (1986)] have shown that the absolute permeability k of a porous medium with a broad distribution of pore sizes is related to a characteristic length l_(c) by: ##EQU16## l_(c) in a percolation model represents the largest pore size such that all pores with a diameter d≧l_(c) form an infinite connected cluster across the pore space. Therefore m and n in the permeability relations 28 and 29 are approximately equal to 2.0 according to the results of mercury injection experiments. In practice the proportionality constants and exponents can be empirically determined in a calibration experiment.

The theory given to explain the present invention is presented for the sake of illustration only and is not intended to necessarily limit the scope of the claimed invention.

EXPERIMENTAL PROCEDURE

The method of this invention was tested in the laboratory using a nuclear magnetic resonance spectrometer whose functionality can replicate the capabilities of an NMR logging tool instrument. The samples used were sandstone core plugs from various geological formations in North America. For all porous samples we used the following procedure to prepare the samples: The samples were placed in a sealed container and imbibed with water. After imbibing a core plug with water for several hours it was taken out, sealed with Teflon tape and transferred to an NMR glass tube for measurements. In between measurements the samples were kept under water in a sealed container.

Several samples of sandstone rock were used in developing the present invention. The permeability and porosity of these samples was determined by standard methods. In addition on samples obtained from the same batch of rock cores mercury injection experiments were performed to determine a characteristic pore (throat) size l_(c). Mercury is a non-wetting fluid and under an applied pressure the mercury will first penetrate the largest pores of the medium. For a certain threshold pressure the first continuous path of mercury will be formed between the two ends of the sample and this will be detectable as a jump in the electrical conductivity across the sample. The length l_(c) can be calculated with the Washburn equation from the threshold pressure at which this first conducting mercury path is established across the sample. The values for permeability, porosity and l_(c) for the set of sandstone samples used for the NMR experiments are listed in Table 1. These values of permeability and l_(c) obtained independently from the NMR measurements will have to be compared with the predictions using T₂ (τ→0).

                  TABLE 1                                                          ______________________________________                                         Sandstone samples and their porosity, permeability and l.sub.c deter-          mined independently of the NMR measurements.                                   sandstone type                                                                            l.sub.c [μm]                                                                         porosity [%]                                                                              permeability [md]                               ______________________________________                                         Berea      14.6     20.5       273                                             Marsing No. 2                                                                             118.0    29.5       54,000                                          Red Navajo 23.5     23.6       1138                                            Nugget     10.77    10.9       4.16                                            Silver No. 1                                                                              23.0     12.2       14.1                                            Layered Navajo                                                                            28.5     25.1       883                                             Marsing No. 1                                                                             78.6     23.9       1276                                            Silver No. 2                                                                              79.2     30.2       21,000                                          Table No. 2                                                                               91.6     24.1       3000                                            ______________________________________                                    

All laboratory NMR experiments were performed at a field strength of 7.05 Tesla corresponding to a Larmor frequency of the hydrogen nucleus of ν_(L) =300.13 MHz . The T₁ measurements were made with a standard inversion recovery pulse sequence. The T₁ measurements were only made for completeness and are not important for the development of this invention. To acquire T₂ data we use the Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence which is comprised of an initial 90 radio frequency pulse which tips the longitudinal magnetization into the transverse plane. It is followed a time τ/2 later by a train of 180 pulses with constant pulse spacing τ. At the midpoint between the 180 pulses the nuclear spin magnetization of stationary spins is refocused and a Hahn echo is formed. The echo will be attenuated for spins moving along the orientation of the magnetic field gradients. The magnetization is sampled at the center of the Hahn echoes. This means that for a train of n 180° pulses we acquire n data points. We repeat this experiment for a set of τ values. This procedure corresponds exactly to one outlined above in the section titled "Method of Invention". FIG. 5 shows a schematic diagram of the pulse sequence.

When measuring the T₂ decay with the CPMG sequence as a function of pulse spacing, T₂ decreases rapidly for porous samples imbibed with water or oil. By using the CPMG pulse sequence we can ascribe this decay of T₂ to the effects of internal magnetic field gradients in the porous sample, which are mainly due to the magnetic susceptibility difference for the fluid imbibing the sample and the matrix. For most samples we recorded the decay of the transverse magnetization with the CPMG sequence for a set of 8-16 values of τ.

DATA ANALYSIS

The T₂ magnetization decays were fit to a stretched exponential function with a nonlinear least squares fitting procedure. FIG. 6 shows an example of the stretched exponential fit to a T₂ CPMG decay curve. The stretched exponential function approximates the experimental data well over 2-3 orders of magnitude as can be seen from FIG. 6b. No data points were discarded before fitting the data to the stretched exponential function.

We acquired for each sample a set of CPMG decay curves by varying τ. The stretched exponential fit yields a value of T₂ for each value of τ. The composite set of data point for T₂ (τ) is shown in FIG. 7 for three representative sandstone samples. We fit the resulting set of T₂ (τ) data to a single exponential function. FIG. 7 shows the nonlinear least squares fits as dotted curves. This allows one to extrapolate T₂ (τ) to τ=0. T₂ (τ→0) should therefore give a value of the transverse magnetization relaxation time T₂ independent of the strength of the internal magnetic field gradients and be proportional to some characteristic length of the pores. FIG. 8 shows a plot of T₂ (τ=0) versus the characteristic pore diameter l_(c) determined from the threshold pressure for mercury injection via the Washburn equation. Such a relationship is of great value for a rapid order of magnitude determination of permeability based on the above mentioned relationship proposed by Katz and Thompson: k=1/126 l_(c) ² φ². And indeed as FIG. 9 shows there exists a correlation between the product of porosity φ and T₂ (τ→0) and the independently determined permeability of the porous rock samples as predicted by the relation k ∝ [T₂ (τ→0)]² φ². In this equation the exponent of T₂ (τ→0) is predicted to be 2. However the range of the data in FIG. 9 would indicate that the exponent falls into a range of about 1.8 to 2.2. T₂ values obtained with the CPMG pulse sequence (which should be the method of choice) are strongly dependent on the pulse spacing τ. Only when the gradient and diffusion induced relaxation is taken into account, do the resulting T₂ (τ) values extrapolated to τ=0 show the discussed correlations with permeability and l_(c).

For the restricted diffusion analysis of T₂ (τ) we used a nonlinear least squares fitting algorithm to fit the experimental T₂ (τ) data to the expression of equation 22. The prefactor of τ in the exponent of the exponential is one of the adjustable parameters. l_(nmr) was directly determined from this adjustable fitting parameter obtained from the nonlinear least squares fit.

The present invention takes advantage of the correlations between T₂ determined with NMR and a characteristic pore or throat size which determines the fluid flow permeability. This characteristic length l_(c) can also be determined by mercury injection but mercury injection experiments cannot be carried out in a down-hole environment. For in situ NMR relaxation time measurements in rock formations with an NMR logging tool it is feasible to carry out a T₂ measurement described here with the present generation of logging tools and predict the fluid flow permeability using NMR in a manner which is much less time-consuming than other methods known to date. 

What is claimed is:
 1. A method for determining the permeability of porous media saturated with a liquid using nuclear magnetic resonance (NMR) and comprising:(a) applying a radiofrequency pulse sequence which after an initial pulse generates successive spin echoes with a train of radio frequency pulses spaced apart by a time interval of length τ wherein all pulses have a carrier frequency corresponding to the Larmor frequency of the fluid spins filling the pore space of the medium for which the fluid flow permeability is to be determined; (b) measuring the decay of the transverse magnetization at each of the successive regularly spaced midpoints between the 180 degree pulses where the midpoints coincide with the peak of the spin echoes; (c) repeating steps a and b at least one more time wherein each repeat of step (a) uses said radio frequency pulse train with a different value of the pulse spacing τ; (d) determining the transverse relaxation time T₂ (τ), from the transverse magnetization decay for each value of τ, and determining one of a prefactor Δ and a restricted diffusion length, l_(nmr), from said T₂ (τ): (e) measuring the porosity of said porous media; (f) determining the permeability of said media from the porosity and one of the prefactor Δ and a restricted diffusion length, l_(nmr), from said T₂ (τ):.
 2. The method of claim 1 wherein said T₂ (τ) is of a form which includes an exponential function of τ.
 3. The method of claim 2 wherein said T₂ (τ) is obtained from an expansion of the exponential functions.
 4. The method of claim 2 wherein said T₂ (τ) is a simple exponential function of τ and said prefactor Δ is obtained by extrapolation of T₂ (τ) to τ→0.
 5. The method of claim 1 wherein said porous media is an earth formation and the measurement is performed in a well-bore environment with an NMR logging tool.
 6. The method of claim 1 wherein said radio frequency pulse sequence is applied according to the Carr-Purcell-Meiboom-Gill (CPMG) sequence.
 7. The method of claim 1 wherein the relaxation time, T₂ is obtained from the transverse magnetization decay in step (d) by relating the transverse magnetization to time by a stretched exponential function.
 8. The method of claim 1 wherein said permeability of step (f) is determined from the product of the porosity and restricted diffusion length l_(nmr).
 9. The method of claim 8 wherein said porosity and restricted diffusion length l_(nmr) are each raised to a power.
 10. The method of claim 8 wherein l_(nmr) is determined using a restricted diffusion model.
 11. The method of claim 1 wherein said permeability of step (f) is determined from the product of the porosity and the prefactor Δ.
 12. The method of claim 11 wherein said porosity and the prefactor Δ are each raised to a power.
 13. The method of claim 11 wherein prefactor Δ is determined as prefactor of the observed T₂ (τ) dependence.
 14. The method of claim 4 wherein said permeability of step (f) is obtained from the product of the porosity and the extrapolated transverse relaxation time, T₂ at τ=0.
 15. The method of claim 14 wherein said porosity and the extrapolated transverse relaxation time, T₂ at τ=0 are each raised to a power.
 16. The method of claim 9 wherein said powers are between about 1.8 and 2.2.
 17. The method of claim 12 wherein said powers are between about 1.8 and 2.2.
 18. The method of claim 15 wherein said powers are between about 1.8 and 2.2.
 19. The method of claim 16 wherein said powers are
 2. 20. The method of claim 17 wherein said powers are
 2. 21. The method of claim 18 wherein said powers are
 2. 